An electric power transmission network comprises high-voltage tie lines and substations for transforming voltages and for switching connections between lines. Loads and power generating plants are connected to the network. An important issue when controlling power generation and load flow is to keep the network stable, i.e. to avoid voltage collapse and swings. Existing SCADA (Supervisory control and data acquisition) systems provide estimates about the stability of a network. However, such an estimate is based on the assumption that the network is in a steady state condition. Consequently, it is not valid if it is obtained during a transient condition, i.e. in the time after a fault or contingency has occurred and before the network is back in a seemingly steady state. It often happens that the network seems to be in a steady state after a contingency, however, an instability caused by the contingency develops unnoticed. The instability is detected by the SCADA system only when the network voltages are severely affected. At this point in time, remedial actions such as load shedding must be drastic, if complete collapse of the network is to be avoided. It therefore is desirable to obtain, after a contingency is detected, an early estimate of the future stability of a network, such that remedial actions can be executed before the effects of the instability become too large.
U.S. Pat. No. 5,638,297 shows a method of on-line transient stability assessment of an electrical power system. A computer model is used to simulate an effect of an artificially introduced study contingency. The simulation uses a step-by-step integration method and predicts future effects of the contingency on the network, in particular on network stability. However, the method requires a full model of dynamic behavior of the network and a significant computational effort for the simulation. The algorithm uses pre-calculations that are made before a given contingency occurs. If a contingency was not pre-calculated or if cascaded contingencies occur, the algorithm fails. If applied to voltage stability, the algorithm would fail as well for cascaded contingencies since an exhaustive pre-calculation of combinations of contingencies is not practicable.